On the Role of Bottom Pressure Torques in Wind-Driven Gyres

2021 ◽  
Vol 51 (5) ◽  
pp. 1441-1464
Author(s):  
Andrew L. Stewart ◽  
James C. McWilliams ◽  
Aviv Solodoch
Keyword(s):  
Wind Stress ◽  
Bottom Friction ◽  
Bottom Pressure ◽  
Wind Stress Curl ◽  
Previous Theory ◽  
Model Experiments ◽  
Basin Scale ◽  
Vorticity Budget ◽  
Gyre Circulation

AbstractPrevious studies have concluded that the wind-input vorticity in ocean gyres is balanced by bottom pressure torques (BPT), when integrated over latitude bands. However, the BPT must vanish when integrated over any area enclosed by an isobath. This constraint raises ambiguities regarding the regions over which BPT should close the vorticity budget, and implies that BPT generated to balance a local wind stress curl necessitates the generation of a compensating, nonlocal BPT and thus nonlocal circulation. This study aims to clarify the role of BPT in wind-driven gyres using an idealized isopycnal model. Experiments performed with a single-signed wind stress curl in an enclosed, sloped basin reveal that BPT balances the winds only when integrated over latitude bands. Integrating over other, dynamically motivated definitions of the gyre, such as barotropic streamlines, yields a balance between wind stress curl and bottom frictional torques. This implies that bottom friction plays a nonnegligible role in structuring the gyre circulation. Nonlocal bottom pressure torques manifest in the form of along-slope pressure gradients associated with a weak basin-scale circulation, and are associated with a transition to a balance between wind stress and bottom friction around the coasts. Finally, a suite of perturbation experiments is used to investigate the dynamics of BPT. To predict the BPT, the authors extend a previous theory that describes propagation of surface pressure signals from the gyre interior toward the coast along planetary potential vorticity contours. This theory is shown to agree closely with the diagnosed contributions to the vorticity budget across the suite of model experiments.

Generation and propagation of 21-day bottom pressure variability driven by wind stress curl in the East China Sea

2020 ◽  
Vol 39 (7) ◽  
pp. 91-106
Author(s):  
Hua Zheng ◽  
Xiao-Hua Zhu ◽  
Hirohiko Nakamura ◽  
Jae-Hun Park ◽  
Chanhyung Jeon ◽  
...  
Keyword(s):  
East China Sea ◽  
Wind Stress ◽  
East China ◽  
Bottom Pressure ◽  
The East China Sea ◽  
Wind Stress Curl ◽  
China Sea

Regimes of Thermocline Scaling: The Interaction of Wind Stress and Surface Buoyancy

2007 ◽  
Vol 37 (8) ◽  
pp. 2009-2021 ◽  
Author(s):  
Paola Cessi
Keyword(s):  
Potential Energy ◽  
Surface Temperature ◽  
Heat Transport ◽  
Wind Stress ◽  
Wind Stress Curl ◽  
Available Potential Energy ◽  
Ekman Flow ◽  
Diapycnal Diffusivity ◽  
Surface Buoyancy

Abstract The role of the relative geometry of mechanical forcing (wind stress) and buoyancy forcing (prescribed surface temperature) in the maintenance of the main thermocline is explored. In particular, the role of the wind stress curl in enhancing or suppressing the generation of baroclinic eddies is studied in simplified domains. The dependence of key quantities, such as the depth of the thermocline and the maximum heat transport, on the external parameters such as diapycnal mixing and dissipation rate is examined. Qualitatively different regimes are found depending on the relative phase of the wind stress and surface buoyancy distribution. The most efficient arrangement for eddy generation has Ekman pumping (suction) in conjunction with high (low) surface buoyancy. This corresponds to the situation found in the midlatitudes, where the surface Ekman flow carries heat toward the warmer region (i.e., upgradient of the surface temperature). In this case, strong eddy fluxes are generated in order to counteract the upgradient heat transport by the Ekman cell. The result is a thermocline whose depth is independent of the diapycnal diffusivity. However, the competition between these opposing heat fluxes leads to a weak net heat transport, proportional to the diffusivity responsible for the diabatic forcing. This arrangement of wind stress provides a large source of available potential energy on which eddies can grow, so the mechanical energy balance for the eddies is consistent with a substantial eddy heat flux. When the same surface temperature distribution is paired with the opposite wind stress curl, the mean flow produces a sink, rather than a source, of available potential energy and eddies are suppressed. With this arrangement, typical of low latitudes and the subpolar regions, the Ekman overturning cell carries heat downgradient of the surface temperature. Thus, the net heat transport is almost entirely due to the Ekman flow and is independent of the diapycnal diffusivity. At the same time the thermocline is a thin, diffusive boundary layer. Quantitative scalings for the thermocline depth and the poleward heat transport in these two limiting cases are contrasted and successfully compared with eddy-resolving computations.

scholarly journals The Role of Back Pressure in the Atmospheric Response to Surface Stress Induced by the Kuroshio

2017 ◽  
Vol 74 (2) ◽  
pp. 597-615 ◽  
Author(s):  
Kohei Takatama ◽  
Niklas Schneider
Keyword(s):  
Boundary Layer ◽  
Wind Stress ◽  
Back Pressure ◽  
Ocean Currents ◽  
Sea Surface ◽  
Ocean Current ◽  
Wind Stress Curl ◽  
General Response ◽  
The Kuroshio

Abstract The effect of ocean current drag on the atmosphere is of interest as a test case for the role of back pressure, because the response is independent of the thermally induced modulation of the boundary layer stability and hydrostatic pressure. The authors use a regional atmospheric model to investigate the impact of drag induced by the Kuroshio in the East China Sea on the overlying winter atmosphere. Ocean currents dominate the wind stress curl compared to the impacts of sea surface temperature (SST) fronts. Wind stress convergences and divergences are weakly enhanced even though the ocean current is almost geostrophic. These modifications change the linear relationships (coupling coefficients) between the wind stress curl/divergence and the SST Laplacian, crosswind, and downwind gradients. Clear signatures of the ocean current impacts are found beyond the sea surface: sea surface pressure (back pressure) decreases near the current axis, and precipitation increases over the downwind region. However, these responses are very small despite strong Ekman pumping due to the current. A linear reduced gravity model is used to explain the boundary layer dynamics. The linear vorticity equation shows that the oceanic influence on wind stress curl is balanced by horizontal advection decoupling the boundary layer from the interior atmosphere. Spectral transfer functions are used to explain the general response of back pressure to geostrophic ocean currents and sea surface height.

scholarly journals Response of Near-Inertial Shear to Wind Stress Curl and Sea Level

2019 ◽  
Vol 9 (1) ◽  
Author(s):  
Jing Gao ◽  
Jianing Wang ◽  
Fan Wang
Keyword(s):  
Sea Level ◽  
Wind Stress ◽  
Deep Ocean ◽  
Wind Stress Curl ◽  
Sea Level Anomalies ◽  
Ocean Response ◽  
Downward Propagation ◽  
Shear Energy ◽  
And Sea Level

AbstractNear-inertial waves (NIWs) contain a pronounced portion of shear energy in the internal wave field and is of great importance to deep ocean mixing. However, accurate simulation of NIWs remains a challenge. Here we analyzed 3-year long mooring observation of velocity profiles over 80–800 m to study the responses of near-inertial downward shear to varying wind stress curls and sea level anomalies (SLAs). It is demonstrated that moderate (even weak) cyclone makes more contributions to enhanced shear below the pycnocline than very strong cyclone. Because very strong curl can stall the downward propagation of large shear. The large positive and negative SLAs cause the accumulation of large shear in the lower and upper parts of the pycnocline through inducing downwelling and upwelling motions, respectively. Time variation of near-inertial shear was strongly influenced by cases of large curls and interannual variation of SLA, and thus did not follow the seasonal variation of wind stress. Our analyses suggest that matched fields of wind stress curl and SLA, and well representing the ocean response to moderate cyclone are needed in simulating the role of NIWs on mixing.

The Antilles Current and wind-driven gyre circulation at 26oN

2021 ◽  
Author(s):  
Eleanor Frajka-Williams ◽  
William E. Johns ◽  
Harry L. Bryden ◽  
David A. Smeed ◽  
Aurelie Duchez ◽  
...  
Keyword(s):  
Interannual Variability ◽  
Wind Stress ◽  
Meridional Overturning Circulation ◽  
Western Boundary ◽  
Florida Current ◽  
Boundary Current ◽  
Wind Stress Curl ◽  
The Bahamas ◽  
Western Atlantic ◽  
Gyre Circulation

<p>The Antilles Current is a narrow, northward flowing boundary current in the western Atlantic just east of the Bahamas. Its role in the larger scale circulation has been debated: alternately thought to be part of the western boundary closure of the gyre circulation or the northward flowing limb of the meridional overturning circulation (MOC). From 19 years of moored current meter observations (1987--1991, 2004--2018), we define the strength of the Antilles Current by the net transport between the Bahamas and 76.5°W (spanning about 45 km zonally) and in the thermocline (0–1000 m). We find a mean northward transport of 3.5 Sv, substantial interannual variability, and no discernable trend since 1987. The interannual variability of the AC transport is independent of the variability of the Florida Current (the Gulf Stream through the Florida Straits). Instead, the Antilles Current contributes to the interannual variability of the MOC at 26°N, while the trend in the strength of the gyre circulation (defined as the transbasin thermocline transport minus the AC) is responsible for the trend in the MOC. In particular, the 2009/10 slowdown of the MOC resulted from a weaker northward AC transport, rather than an intensified gyre transport. Using the recent 14 years of in situ transport records, we compare the interannual variability of the gyre circulation to that of wind stress curl forcing via a Sverdrup transport calculation, identifying a potential role for wind stress curl (WSC) forcing at 26°N with a ~2 year lag until 2016.<span>  </span>From 2016, the predicted gyre circulation using WSC diverges from the measured gyre strength.</p>

scholarly journals ENSO-correlated fluctuations in ocean bottom pressure and wind-stress curl in the North Pacific

Ocean Science ◽  
2011 ◽  
Vol 7 (5) ◽  
pp. 685-692 ◽  
Author(s):  
D. P. Chambers
Keyword(s):  
Wind Stress ◽  
North Pacific ◽  
Low Frequency ◽  
Ocean Model ◽  
Ocean Bottom ◽  
Bottom Pressure ◽  
Wind Stress Curl ◽  
Ocean Bottom Pressure ◽  
The North ◽  
The North Pacific

Abstract. We examine the output of an ocean model forced by ECMWF winds to study the theoretical relationship between wind-induced changes in ocean bottom pressure in the North Pacific between 1992 until 2010 and ENSO. Our analysis indicates that while there are significant fluctuations correlated with some El Niño and La Niña events, the correlation is still relatively low. Moreover, the ENSO-correlated variability explains only 50 % of the non-seasonal, low-frequency variance. There are significant residual fluctuations in both wind-stress curl and ocean bottom pressure in the region with periods of 4-years and longer. One such fluctuation began in late 2002 and has been observed by the Gravity Recovery and Climate Experiment (GRACE). Even after accounting for possible ENSO-correlated variations, there is a significant trend in ocean bottom pressure in the region, equivalent to 0.7 ± 0.3 cm yr−1 of sea level from January 2003 until December 2008, which is confirmed with steric-corrected altimetry. Although this low-frequency fluctuation does not appear in the ocean model, we show that ECMWF winds have a significantly reduced trend that is inconsistent with satellite observations over the same time period, and so it appears that the difference is due to a forcing error in the model and not an intrinsic error.

An Idealized Model of Ocean Gyres near Pine Island Ice Shelf and Thwaites Ice Shelf

2021 ◽  
Author(s):  
Yixi Zheng ◽  
David Stevens ◽  
Karen Heywood ◽  
Benjamin Webber ◽  
Bastien Queste
Keyword(s):  
Sea Ice ◽  
Wind Stress ◽  
Numerical Models ◽  
Oceanic Circulation ◽  
Wind Fields ◽  
Ice Shelf ◽  
Wind Stress Curl ◽  
Ice Shelves ◽  
Ice Conditions ◽  
Gyre Circulation

<p>Floating ice shelves buttress the Antarctic Ice Sheet, which is losing mass rapidly mainly due to oceanic melting and the associated disruption to glacial dynamics. The local oceanic circulation near ice shelves is therefore important for the prediction of future ice mass loss and related sea-level rise as it determines the water mass exchange, heat transport under the ice shelf, and the resultant melting. However, the dynamics controlling the near-coastal circulation are not fully understood, particularly relating to seasonal and interannual changes in wind stress curl and ice cover. A gyre circulation (27 km radius, cyclonic) in front of the Pine Island Ice Shelf has been identified in both numerical models and velocity observations. In 2019 in the west of Thwaites Ice Shelf, for the first time in this habitually ice-covered region, another gyre circulation rotating in a different direction (13 km, anticyclonic) was detected by velocity observations. Here we use an idealised configuration of MITgcm, with idealised forcing based on ERA-5 climatological wind fields and simplified sea ice conditions from MODIS satellite images, to reproduce key features of the observed gyres near Pine Island Ice Shelf and Thwaites Ice Shelf. A barotropic version of the model is able to reproduce the gyres driven solely by the wind. We show that the modelled gyre direction depends upon the angle between the wind direction and the sea ice front. Gyres generated by wind in sea-ice-free conditions have directions controlled by the wind stress curl. When sea ice is present, the wind stress exerted on the sea surface is reduced, leading to a modified wind stress curl and a resultant change in gyre direction.</p>

Wind Spatial Variability and Topographic Wave Frequency

2008 ◽  
Vol 38 (9) ◽  
pp. 2085-2096 ◽  
Author(s):  
Elad Shilo ◽  
Yosef Ashkenazy ◽  
Alon Rimmer ◽  
Shmuel Assouline ◽  
Yitzhaq Mahrer
Keyword(s):  
Spatial Variability ◽  
Wind Stress ◽  
Numerical Solutions ◽  
Rotation Frequency ◽  
Wind Stress Curl ◽  
Elliptic Paraboloid ◽  
Natural Lakes ◽  
Nonlinear Shallow Water Equations ◽  
Double Gyre

Abstract The association of topographic waves with wind action has been documented in several natural lakes throughout the world. However, the influence of the wind’s spatial variability (wind stress curl) on the frequency of topographic waves has only been partially investigated. Here the role of wind stress curl on the frequency of topographic waves in an idealized elliptic paraboloid basin has been studied both analytically and numerically. It is shown that the analytical solution is the sum of an elliptic rotation determined by the wind stress curl and two counterrotating circulation cells, which propagate cyclonically after the wind ceases. Furthermore, it is shown that cyclonic elliptical rotation (associated with positive wind stress curl) increases the rotation frequency of the double-gyre pattern while anticyclonic elliptical rotation (associated with negative wind stress curl) decreases the oscillatory mode frequency. It is also shown that bottom friction has some effect on the structure of the double-gyre pattern but hardly affects the oscillatory frequency. Numerical solutions of the depth-integrated nonlinear shallow-water equations confirmed that the frequency of the topographic wave increases (decreases) when forcing the model with cyclonic (anticyclonic) wind curl.

The sensitivity of Southeast Pacific heat content to changes in ocean structure

2021 ◽  
Author(s):  
Dan Jones ◽  
Emma Boland ◽  
Andrew Meijers ◽  
Gael Forget ◽  
Simon Josey ◽  
...  
Keyword(s):  
Objective Function ◽  
Wind Stress ◽  
Heat Content ◽  
Global Ocean ◽  
Wind Stress Curl ◽  
Adjoint Sensitivity ◽  
Mode Water ◽  
Near Surface ◽  
Basin Scale ◽  
Southeast Pacific

<p>The Southern Ocean features ventilation pathways that transport surface waters into the subsurface thermocline on timescales from decades to centuries, sequestering anomalies of heat and carbon away from the atmosphere and thereby regulating the rate of surface warming. Despite its importance for climate sensitivity, the factors that control the distribution of heat along these pathways are not well understood. In this study, we use an observationally-constrained, physically-consistent global ocean state estimate (i.e. ECCOv4) to examine how changes in ocean properties can affect the heat content both in the mixed layer and in the recently ventilated subsurface, focusing on the Southeast Pacific. First, we carry out a comprehensive adjoint sensitivity study using near-surface heat content as the objective function, highlighting the locations and timescales with the largest potential to affect the properties of relevant subduction regions. Next, we use a set of numerical tracer release experiments to identify the subduction and export pathways from the surface into the subsurface thermocline, thereby defining the recently ventilated interior. Using the tracer distribution to define our objective function, we employ an adjoint method to calculate temporally-evolving sensitivity maps that highlight the processes, locations, and timescales that are potentially most relevant for changing the heat content of the recently ventilated Pacific. In order to examine the full nonlinear response, we use the adjoint sensitivity fields to design a set of forward, nonlinear perturbation experiments. We find surprisingly weak sensitivities to high latitude wind stress and heat flux, and relatively high sensitivities to wind stress curl in subpolar latitudes. Despite the localized nature of mode water subduction hotspots, changes in basin-scale density gradients are an important controlling factor on heat distribution in the Southeast Pacific.</p>

scholarly journals ENSO-correlated fluctuations in ocean bottom pressure and wind-stress curl in the North Pacific

2011 ◽  
Vol 8 (4) ◽  
pp. 1631-1655
Author(s):  
D. P. Chambers
Keyword(s):  
Wind Stress ◽  
North Pacific ◽  
Low Frequency ◽  
Satellite Observations ◽  
Ocean Bottom ◽  
Bottom Pressure ◽  
Wind Stress Curl ◽  
Ocean Bottom Pressure ◽  
The North ◽  
The North Pacific

Abstract. We examine the magnitude of ENSO-correlated variations in wind-stress curl and ocean bottom pressure in the North Pacific between 1992 until 2010, using satellite observations and model output. Our analysis indicates that while there are significant fluctuations correlated with some El Niño and La Niña events, the correlation is still relatively low. Moreover, the ENSO-correlated variability explains only 50 % of the non-seasonal, low-frequency variance. There are significant residual fluctuations in both wind-stress curl and ocean bottom pressure in the region with periods of 4-years and longer. One such fluctuation began in late 2002 and has been observed by the Gravity Recovery and Climate Experiment (GRACE). Even after accounting for ENSO variations, there is a significant trend in ocean bottom pressure in the region, equivalent to 0.7 ± 0.3 cm yr−1 of sea level from January 2003 until December 2008, which is confirmed with steric-corrected altimetry. Although this low-frequency fluctuation does not appear in an ocean model, we show that the winds used to force the model have a significantly reduced trend that is inconsistent with satellite observations over the same time period.

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